RAID Rebuild: Why It Can Lead to Loss of All Data
RAID Rebuild: Why It Can Lead to Loss of All Data
You replaced the failed drive and started the rebuild. The progress indicator shows 47%. And then... another drive fails. All data is lost.
This is not a nightmare. It's a real scenario that happens more often than it should. A RAID rebuild, which is supposed to restore your data's protection, is paradoxically one of the riskiest processes your data can go through.
What Is a RAID Rebuild
Definition
A RAID rebuild is the process of restoring redundancy after a drive failure. The controller reads data and parity from the remaining drives and calculates the missing data onto the new drive.
How It Works
RAID 5:
- The controller reads every sector from the healthy drives
- For each stripe it calculates:
New sector = Disk1 XOR Disk2 XOR ... XOR Parity - It writes the result to the new drive
RAID 6: The same principle, but using two independent parities.
RAID 1/10: Simpler – a plain copy from the mirror drive.
Duration
| RAID capacity | Approximate rebuild time |
|---|---|
| 1 TB | 2-4 hours |
| 4 TB | 8-16 hours |
| 12 TB | 24-48 hours |
| 24 TB+ | 2-4 days |
Depends on drive speed, the controller and the load.
Why a Rebuild Is Risky
A Stress Test for the Remaining Drives
During a rebuild, the controller has to read every sector of the remaining drives. That means reading their entire capacity – something that never happens in normal operation.
What it means:
- 100% I/O utilisation
- Higher drive temperatures
- Mechanical stress (on HDDs)
- Latent defects coming to light
Hidden Problems Come to Light
Some sectors haven't been read for months or even years. They may have degraded, but normal operation doesn't notice – nobody uses the files stored in them.
A rebuild reads everything. And it finds problems you didn't know about.
URE – Unrecoverable Read Error
This is the key concept for understanding rebuild risks.
URE: The Silent RAID Killer
What Is a URE
An Unrecoverable Read Error is a read error the drive cannot correct. The sector remains unreadable even after repeated attempts.
How Often It Occurs
Every drive has a URE rate specification – the probability of an unrecoverable error:
| Drive type | URE rate |
|---|---|
| Consumer HDD | 1 in 10^14 bits |
| Enterprise HDD | 1 in 10^15 bits |
| Enterprise SSD | 1 in 10^17 bits |
The Maths – Why It's a Problem
Let's calculate the probability of a URE during a 12 TB RAID 5 rebuild with consumer drives:
12 TB = 12 × 10^12 bytes = 96 × 10^12 bits
URE rate = 10^14 bits per error
Probability of NO error when reading 12 TB:
P(OK) = (1 - 1/10^14)^(96×10^12) ≈ e^(-0.96) ≈ 38%
Probability of at least 1 URE:
P(URE) ≈ 62%
With 12 TB of consumer drives, there's roughly a 60% chance of a URE during a full read.
Consequences for RAID 5
In RAID 5, a single URE during the rebuild means the entire rebuild fails. The controller has no way to calculate the missing data if one of the input sectors is unreadable.
Result: The array stays degraded, the rebuild fails, and if another drive fails – all data is lost.
Why RAID 6 Is Safer
RAID 6 has two independent parities. A single URE during the rebuild is not a problem – the controller can calculate the data from the second parity.
That's why we recommend RAID 6 for:
- Large arrays (6+ drives)
- Large drives (4 TB+)
- Consumer drives (worse URE rate)
RAID configuration comparison →
Probability of Failure During a Rebuild
Risk Table
| Situation | Failure probability |
|---|---|
| RAID 5, 4×1TB, new drives | ~1-5% |
| RAID 5, 4×4TB, 3 years | ~10-20% |
| RAID 5, 8×8TB, 4 years | ~30-40% |
| RAID 5, 8×12TB, 5 years | ~40-60% |
| RAID 6, 8×12TB, 5 years | ~5-15% |
Factors That Increase the Risk
Drive age: Older drives = more wear = a higher probability of UREs and failure.
Drive size: Larger drives = more data to read = a higher probability of a URE.
Number of drives: More drives = more potential points of failure.
SMART warnings: Drives with warnings are significantly more likely to fail during a rebuild.
Hot Spare – Solution or Illusion?
What Is a Hot Spare
A spare drive connected to the RAID array but not in use. When a drive fails, it automatically replaces the failed drive and starts the rebuild.
Advantages
Automatic start: No waiting for a new drive – the rebuild starts immediately.
Shorter degraded period: A smaller window during which the array is vulnerable.
Disadvantages
The rebuild is still risky: A hot spare doesn't reduce the rebuild risks – UREs, the domino effect, stress on the drives.
A false sense of security: "We have a hot spare, we're safe." No – you just reach the rebuild phase faster.
Cost: A drive that normally does nothing.
Recommendation
A hot spare – YES, but be aware of its limits. It complements backups; it doesn't replace them.
The Correct Rebuild Procedure
Before the Rebuild
1. Full backup (if possible) If the array is readable, back up your critical data. It's your insurance in case the rebuild fails.
2. SMART check of all drives Check the SMART values of the remaining drives:
- Reallocated Sector Count
- Current Pending Sector
- Spin Retry Count
If any drive shows warnings, don't rebuild – professional recovery is the better option.
3. Documentation Record:
- Drive models and serial numbers
- Drive positions
- RAID configuration
- SMART values
4. Plan B What will you do if the rebuild fails? Have a professional data recovery lab lined up.
During the Rebuild
1. Minimise I/O Shut down applications that use the RAID. Less load = lower risk.
2. Monitoring Monitor the progress and drive temperatures. High temperature = risk.
3. Be prepared for failure If the rebuild fails or errors appear, stop immediately and get help.
After the Rebuild
1. Verify integrity Run a consistency check (scrub) if the controller supports it.
2. Test your backup Verify that the backup is current and working.
3. SMART check Check the SMART values again – the rebuild may have revealed latent problems.
Alternatives to a Rebuild
Professional Recovery
Instead of a risky rebuild, the data can be recovered professionally:
- A sector-by-sector copy of each drive
- Virtual RAID reconstruction
- Work on copies, not the originals
Advantages:
- Safer (we don't work on the originals)
- Recovery is possible even after multiple drive failures
- Expert diagnostics
Disadvantages:
- Cost
- Time (days instead of hours)
Restore from Backup
The safest option. If you have a working backup:
- Create a new RAID array
- Restore the data from the backup
- Done
This is why having backups matters.
Upgrade to RAID 6
If you have to deal with the failure anyway, consider an upgrade:
- A new controller that supports RAID 6
- New drives (from different batches)
- Data migration from the backup
When It's Better Not to Rebuild
More Than One Drive with a SMART Warning
If any of the remaining drives shows SMART warnings, a rebuild is a gamble. Professional recovery is safer.
Very Old Drives (5+ Years)
With old drives, the probability of UREs and a domino failure is very high. Consider recovery instead of a rebuild.
Critical Data Without a Backup
If you have no backup and the data is critical, a rebuild is too risky. Professional recovery is the only safe path.
A Previous Attempt Has Failed
If the first rebuild failed, a second attempt has even less chance. The drives are even more worn. Get professionals involved.
What to do in a degraded state →
FAQ
How long does a rebuild take?
It depends on capacity, drive speed and load. Approximately:
- 4 TB: 8-16 hours
- 8 TB: 16-32 hours
- 12 TB+: 1-3 days
Can I use the server during a rebuild?
You can, but you'll slow the rebuild down and increase the risk. For critical data, we recommend keeping activity to a minimum.
Is a rebuild on SSDs safer?
Yes. SSDs have a better URE rate (10^17 vs 10^14) and aren't prone to mechanical failure. The rebuild is faster and less risky.
The rebuild failed – what now?
Stop any further attempts immediately. The drives are in worse condition than before the rebuild. Contact a professional data recovery lab.
Need a Safer Solution?
If your RAID is degraded and you're wary of a rebuild, we can help. Professional recovery is safer than a risky rebuild.
Register your case online – the initial assessment is free and you receive a binding quote before any recovery work starts. No Data, No Fee. Send the drives to our lab with a tracked, insured carrier, hand them in personally in Prague, Vienna or Bratislava, or book our optional insured DPD pickup (€45, non-refundable; available in AT, BE, CZ, DE, DK, FI, FR mainland, HU, IT mainland, LU, NL, PL, SE, SI and SK).